Method for determining the axial load on an elongated member

ABSTRACT

A method for determining the axial load on an elongate member by measuring the differences in time of flight for transversely and longitudinally ultrasonic waves introduced in the elongate member under a zero load condition and at a current temperature and the time of flight for both of these types of waves introduced into the elongate member under the actual load condition, and calculating from these differences in the time of flight the actual load on the elongate member taking into account the influence of any occurring difference in temperature of the elongate member between the zero load condition time of flight measurement and the actual load condition time of flight measurement.

This invention relates to a method for determining the axial load on anelongate member by means of ultrasonic waves.

In particular, the invention concerns a load determining method in whichlongitudinally as well as transversely directed ultrasonic waves areintroduced at one end of an elongate member and the time of flight forthese waves is individually measured.

Such a method is described in U.S. Pat. No. 4,602,511. This known methodis based on the fact that the propagation speed dependency on stress isdifferent between longitudinal waves and transverse waves.

A problem concerned with the above technique is that the time of flightvaries with changing temperature in the object being inspected. The timeof flight varies due to the fact that the propagation speed and thelength of the object vary with the temperature. This results in anundesireable temperature related deviation in the calculated load actingon the object.

This is a problem in particular in assembling screw joints, because thetemperature of the screw may differ to a large extent in the productionenvironment. It is also very difficult to measure the temperature oneach screw in mass production, and the tightening process itselfgenerates heat in the screw.

A recent study verifies that there is a difference between longitudinaland transverse waves as regards the time of flight dependency on thetemperature in the object being inspected. It has been found that thetime of flight of transverse waves is more affected by temperaturechanges than longitudinal waves, whereas, oppositely, the time of flightfor longitudinal waves is more dependent on the load magnitude than thetime of flight for transverse waves.

This means that measured changes in flight time for the longitudinal andtransverse waves provide information from which changes in temperatureand load can be calculated.

The invention comprises a method for determining the actual load on anelongate member by measuring during a first, preliminary zero-loadinspection stage the time of flight for longitudinal and transverseultrasonic waves introduced into the elongate member for determining thezero-load length of the elongate member by measuring the time of flight(TOF) for both types of waves measuring during a second inspection stagethe time of flight (TOF) for both type of waves under the actual loadand temperature condition of the elongate member, comparing the time offlight (TOF) for the introduced waves measured at the zero-loadcondition with the time of flight (TOF) measured at the actual load andtemperature condition, and calculating the actual load magnitude undercompensation for the difference in temperature between the actual loadand zero-load conditions.

In a practical application of the method according to the invention thetransverse and longitudinal waves are introduced into the elongatemember by means of an ultrasonic transducer of the type which ispermanently attached to an end surface of the elongate member. Such atransducer is described in U.S. Pat. No. 5,205,176.

For carrying out the method according to the invention, there is neededthe following physical data of the elongate member:

T₀ (K) An arbitrary temperature at which v_(LC) and v_(S0) are measured,should be set to about room temperature.

v_(L0) (m/s) Velocity of the longitudinal wave at temperature T₀.

v_(S0) (m/s) Velocity of the transverse (shear) wave at temperature T₀.

K_(ST) (s/Km)Relative time of flight change for the transverse wave dueto a change in temperature.

K_(LT) (s/Km)Relative time of flight change for the longitudinal wavedue to a change in temperature.

K_(SL) (s/Nm) Relative time of flight change for the transverse wave dueto a change in tensile load.

K_(LL) (s/Nm) Relative time of flight change for the longitudinal wavedue to a change in tensile load.

The above mentioned data must be collected for each application as theydepend on both the material properties and the geometry (area) of forinstance a screw in a screw joint application.

The following data measured at the time for the tightening:

t_(S0) (s) Zer o load TOF for the transverse wave.

t_(L0) (s) Zero load TOF for the longitudinal wave.

t_(SL) (s) TOF for the transverse wave in the loaded case.

t_(LL) (s) TOF for the longitudinal wave in the loaded case.

and the following may be calculated therefrom:

ΔT (K) Deviation in temperature from T₀ at zero-load.

ΔT_(L) (K) Deviation in temperature from T_(C) in the loaded case.

L (N) The tensile load in the fastener.

l₀ (m) Then length of the screw in zero-load at T₀.

In the initial stage of the tightening of a screw joint a zero-loadmeasurement is made. From that information it is possible to determinethe length (and the temperature) of the screw as follows:

Input v_(L0) v_(S0) K_(ST) K_(LT) t_(S0) t_(L0) L(=0)

Output: l₀ ΔT

Solution:

The zero-load TOF (t_(S0) and t_(L0)) must be equal to the zero-load TOFat T₀ with a temperature correction factor added: ##EQU1## ΔT is thenextracted from (1): ##EQU2## (3) in (2) gives: ##EQU3## l₀ is thenextracted from (4): ##EQU4##

If the actual temperature is needed it can be obtained from T₀ +ΔT whereΔT is extracted from (5) in (3): ##EQU5##

The next step is to calculate the axial load on the screw using thelength and the TOF data. As we know the original length of the screw wecan make a new temperature compensation, thereby eliminating the problemwith heating due to tightening.

Input: v_(L0) v_(S0) K_(ST) K_(LT) K_(SL) K_(LL) l₀ t_(SL) t_(LL)

Output: L ΔT_(L)

Solution:

The difference in TOF from a zero-load measurement at T₀ can be dividedinto a load induced change and a temperature induced change, hence:##EQU6##

From (7) ΔT is extracted: ##EQU7##

From (9) in (8) the actual load on the screw can be extracted: ##EQU8##

The method according to the invention is suitable for but not limited toscrew tightening applications. It may also be useful in:

All types of ultrasonic stress measurements where compensation fortemperature changes is important.

Monitoring stress and temperature simultaneously in safety applicationslike:

pressure vessels

nuclear reactors etc.

In screw tightening applications, the described method may be use tocontrol the power wrench during the tightening process. The ultrasonictransducer used for introducing the transverse and longitudinal waves inthe screw is connected to an electronic operation control unitassociated with the screw tightening tool.

In practical use of the invention as described above, it is desireableto use an ultrasonic transducer of the type being permanently attachedto the end surface of the screw.

Alternatively, the method may be used for inspection of the prevailingload in previously tightened screw joints.

I claim:
 1. Method for determining the axial load on an elongate memberfor which the physical data are registered, comprising the stepsofintroducing longitudinally as well as transversely directed ultrasonicwaves at one end of the elongate member under a zero-load condition andat a current temperature, measuring individually the times of flight forsaid longitudinally and transversely directed waves through the elongatemember at said zero-load condition, and calculating from the physicaldata of the elongate member and from the measured times of flight ofsaid longitudinal and transverse waves the nominal length of theelongate member, introducing longitudinal as well as transverse wavesinto the elongate member under the actual load condition, comparing saidtimes of flight measured under the actual load condition with said timesof flight measured under said zero-load condition, and determining fromthe difference in the wave flight times the actual load on the elongatemember taking into account the influence of any occurring difference intemperature of the elongate member between said zero-load measurementoccasion and the actual load measurement occasion by the equation:##EQU9## (where v_(L0) is the velocity of the longitudinal waves attemperature T₀,v_(S0) is the velocity of the transverse (shear) waves attemperature T₀, K_(ST) is the relative time of flight change for thetransverse waves due to a change in temperature, K_(LT) is the relativetime of flight change for the longitudinal waves due to a change intemperature, K_(SL) is the relative time of flight change for thetransverse waves due to a change in tensile load, K_(LL) is the relativetime of flight change for the longitudinal waves due to a change intensile load, t_(SL) is the time of flight for the transverse waves inthe actual load condition, t_(LL) is the time of flight for thelongitudinal waves in the actual load condition, l₀ is the length of theelongate member in the zero-load condition at the temperature T₀). 2.Method according to claim 1, wherein said transverse and longitudinalwaves are introduced into the elongate member by means of an ultrasonictransducer which permanently attached to an end surface of the elongatemember.
 3. Method according to claim 2, wherein said longitudinal andtransverse waves are continuously or repeatedly introduced in one ormore elongate members forming part of a joint, furthercomprising:measuring continuously or repeatedly the time of flight ofsaid longitudinally and transversely directed waves, and comparing saidtime of flight measurements with preceding time of flight measurementsfor monitoring continuously or repeatedly occurring changes in theactual load on said elongate member or members at varying temperatures.4. Method according to claim 1 for determining the axial load on a screwin a threaded joint, wherein an ultrasonic transducer is coupled on onehand to said screw for introducing therein said transverse andlongitudinal waves and on the other hand to an electronic operationcontrol unit of a screw tightening tool including means for determiningthe actual load on said screw during tightening.
 5. Method according toclaim 4, wherein said transverse and longitudinal waves are introducedinto said screw by means of an ultrasonic transducer which ispermanently attached to an end surface of said screw.
 6. Methodaccording to claim 1, wherein said longitudinal and transverse waves arecontinuously or repeatedly introduced in one or more elongate membersforming part of a joint, further comprising:measuring continuously orrepeatedly the time of flight of said longitudinally and transverselydirected waves, and comparing said time of flight measurements withpreceding time of flight measurements for monitoring continuously orrepeatedly occurring changes in the actual load on said elongate memberor members at varying temperatures.